X-ray computed tomography (CT) is an important imaging tool in modern hospitals and clinics, and it is still under rapid development. Two areas that are of particular concern are radiation dose reduction and multi-energy imaging, and these two areas are interconnected. CT radiation dose is a public major concern, especially for children. A British study quantified the cancer risk associated with the use of diagnostic x-rays, arguably causing about 700 cases of cancer per year in Britain and >5,600 cases in US. Hence, the well-known ALARA (“As Low As Reasonably Achievable”) principle has been accepted. Also, a more recent Image Gently® campaign (www.imagegently.org) has generated a positive impact for all patients. Hence, it is ideal to send X-ray photons along each path as seldom as possible.
CT contrast resolution is rather poor in the context of soft tissue imaging. X-ray detection technology has been almost exclusively based on energy-integration. On the other hand, the best photon-counting detectors recognize photons individually and spectrally. Photon-counting detectors can reveal elemental composition and support contrast-enhanced studies through K-edge imaging. Medipix is a series of state-of-the-art photon-counting detectors for X-ray imaging. However, the dynamic range of the photon-counting detector is rather limited. When the flux on the detector is lower than the maximum count rate, the imaging performance can be good, but if multiple photons arrive in temporal proximity, the detector may not be able to resolve them as separate events. This loss results in spectral distortion.